Immunological detection is currently the method of choice for ligand detection in the field of diagnostics. However, traditional immunological detection methods, such as the enzyme-linked immunosorbent assay (ELISA), suffer from a lack of sensitivity and/or specificity when it comes to the detection of ligands present at a very low concentration. Standard ELISA has a maximum sensitivity of approximately 10 000 cells or copies of target per unit. Normally the sensitivity is not above 100 000 cells or targets. Concentration of proteins by immunomagnetic methods may result in an increase in sensitivity to detect down to one human cell in research studies, but this is largely due to the large size of human cells. Maximum sensitivity for the detection of bacterial cells remains of the order of 1000 cells, even with the use of immunomagnetic antibody methods. Thus there remains a need for more sensitive methods of immunological detection.
U.S. Pat. No. 5,665,539 describes an “immuno-polymerase chain reaction” method for immunological detection. This method is based on the use of a specific antibody conjugated to a double-stranded DNA molecule. Complexes formed by binding of the antibody conjugate to an antigen are detected by first amplifying a region of the double-stranded DNA using PCR and then detecting the amplification products.
The present inventors have developed an alternative method for sensitive antigen (ligand) detection which is based on real-time amplification of a nucleic acid marker. This method combines the specificity of immunological reactions with the sensitivity of nucleic acid amplification and can be used for real-time quantitative measurement.